This invention relates to an improved furnace system for the melting of reactive metals, and, in particular, the melting of aluminum and aluminum alloys. Conventional practice in the melting of aluminum and aluminum alloys generally comprises placing solid metal to be melted, such as pigs, ingots, heavy scrap and the like, in a fuel-fired furnace containing a heel of molten metal. This melting practice was characterized by a low melt rate and extremely high melt losses due to metal oxidation. After the melting has been completed and alloying constitutuents added, the melt is stirred and then transported to a casting facility or to a holding furnace for subsequent casting. During the initial stages of this conventional practice, the melting rates are quite high due to the direct exposure of the solidified metal to the flame and combustion products, but concomitantly, the metal oxidation rate is quite high. It should be noted that metal lost to oxidation is probably the largest single cost in converting the solid metal to molten metal. When most of the solid metal is below the surface of the molten metal, the melt rate is drastically reduced due to the layer of oxide skim which inherently forms on the molten metal surface and the formation of a slushy aluminum at the solid-liquid metal interface both of which severely lower the heat transfer rate.
The metal products with a high surface-to-weight ratio which characteristically generate much oxide when melted by normal practice, such as thin sheet, foil, chips and the like, are usually melted in separate melting facilities, cast into pigs or sows, then remelted in the method described above.
The suggestion has been made (see, for example, U.S. Pat. No. 3,276,758 - Baker et al) to charge small chips and the like into a separate bay while pumping molten metal from a heating hearth to the separate bay to quickly submerge the chips and the like into the molten metal and melt the solid charge. The molten metal in the charging bay is returned to the heating hearth by gravity. However, one major problem with this system is the lack of a homogeneous melt. The melt in the heating hearth tends to stratify and frequently the cooler molten metal returned to the heating hearth will short circuit to the pump intake port.
Against this background, the present invention was developed.